自花授粉 self-pollination 三系杂交稻 three-line method /CMS system 两系杂交稻 two-line method / PGMS and TGMS system 一系杂交稻 one-line method / apomixis system 杂种优势 heterosis 品种间杂交 intervarietal hybrids 亚种间杂